It is known in the art relating to crankshafts for internal combustion engines to make one-piece castings or forgings which entails considerable machining to remove large quantities of excess metal to define crank pins, main journals, and crank arms which may include counterweights. When the crankshaft is intended for use in a two stroke engine, seal plates must also be provided which requires substantially more machining if the entire crankshaft is to remain as one piece, or otherwise the seal plates must somehow be separately assembled.
It is also known that by assembling the crankshaft of many separate elements much of the machining can be eliminated and the remaining machining is performed on small simple elements such as pins, journals, crank arms and seal plates. The elements are then joined by bolting together or by press fit, for example, to assemble the entire crankshaft. The best way of joining the elements depends on the particular application or load requirements of the assembly. One type of joint, such as main journals to crank arms connection, may be assembled by one method while the crank pins, for example, could be joined to crank arms by another method.
A simple and effective method of joining parts on a common shaft is shown in U.S. Pat. No. 4,835,832 to Arnold et. al., entitled "Method of Assembling Tubular Shaft Assemblies", which is incorporated herein by reference. This method, sometimes known as "ballizing", comprises positioning hollow parts, such as cams of a camshaft, on a tube and expanding the tube to hold the parts by clamping the ends of the tube to prevent longitudinal growth and forcing a ball through the tube which is larger than the original tube inner diameter. The hollow parts have a lobular inner cross section to receive the expanded tube outer diameter and include splines to hold the part against rotation on the tube.
Development work on extending the ballizing method to the joining of crankshaft parts has led to a second generation ballizing method which is improved, at least for some applications, over that disclosed in U.S. Pat. No. 4,835,832. This improved joining method can be used advantageously for joining crank pins and/or main journals to crank arms, yielding an assembled crankshaft with a new and improved joint structure.
An additional advantage of an assembled crankshaft over a one-piece crankshaft is that oil passage design is more flexible, using shorter passages and passages which are normal to the surface at entry or exit points, thus minimizing the passage aperture at bearing surfaces. It has previously been proposed to form oil passages in main journal and crankpins which exit at the end of the journal or pin adjacent the crank arms, and to form a diagonal oil passage through the crank arms which connects the journal and pin passages. To make the connection, the parts must be precisely indexed upon assembly to align each of the bearing passage and the crank pin passage with the diagonal passage in the crank arms.